In both the movie and the novel, a Bo hostage guaranteed Michael's safety when he met with Sol and Mac. The novel says that, in addition, Felix Bo enabled Michael's freedom from prosecution for the two murders when the eturned by confessing to the murders.. Novel also says the Bo's supplied hostages for all the families attending the Commission meeting.
As Lou said: hell of a way to make a living. But, the Bo's made a hell of a good sidestory, and added some Sicilian "color."
Incidentally, there was a real-life Felix Bocchicchio. He had a rap sheet as long as your arm and was in the boxing racket with Frankie (Mr. Gray") Carbo.